This invention relates to an improved bandwidth switching circuit for an intermediate frequency amplifier stage in an FM receiver.
The bandwidth of an intermediate frequency amplifier stage in an FM receiver is determined with consideration for selectivity, distortion and other characteristics. Narrowing the bandwitdth improves selectivity and thus substantially attenuates interference from a nearby station, but increases distortion of demodulation signals. In contrast, the wider bandwidth attentuates demodulation signal distortion and improves tone quality, but renders the receiver more susceptible to interference. In order to make effective use of these conflicting characteristics of the narrow and wide bandwidths, a bandwidth switching circuit for an intermediate frequency amplifier stage as shown in FIG. 1 has heretofore been proposed. More specifically, there are provided a narrow-band intermediate frequency amplifier 9 and a wide-band intermediate frequency amplifier 10, either of which can be selected by a change-over switch 11, as an intermediate frequency amplifier stage 5 in an FM receiver comprising an antenna 1, radio frequency amplifier stage 2, local oscillator 3, mixer 4, intermediate frequency amplifier stage 5, FM detector 6, audio frequency amplifier stage 7, and speaker 8. Under a good, interference-free radio field condition, the wide-band intermediate frequency amplifier 10 is selectively connected by the change-over switch 11, whereby distortion of demodulation signal is reduced and good tone quality is obtained. Under a bad radio interference condition, the narrow-band intermediate frequency amplifier 9 is connected by the change-over switch 11 to permit clear reception of desired signals, eliminating the interference.
The change-over switch 11 in the conventional receivers, however, has been manually operated according to the radio field conditions.